1. Field of the Invention
Our invention is a method of continuously measuring airborne mine radiation working level exposure.
2. Description of the Prior Art
Airborne mine radiation has previously been measured or determined by a variety of different methods. The three United States Patents bearing numbers 3,291,986 (Lamb), 3,409,771 (Riel), and 4,055,762 (Durkin) represent the closest known prior art. Of these three references only the Durkin patent is used to arrive at the same result as our invention which is to continuously determine working level exposure of uranium miners to the chain of short-lived daughter products of the gas radon-222. Above an established working level the alpha particles from some of these daughter products are believed to cause lung cancer when deposited in the respiratory system of miners. Like our invention, the Durkin patent determines exposure by forcing ambient air through a filter before it reaches a detector. However, unlike our detector the Durkin invention uses a solid state surface detector to directly measure the count of incident alpha particles from the daughters. In contrast we employ an open face filter and an indirect method of determining the alpha radiation by actually measuring the beta radiation incident on a detector and then, based thereon, calculating the exposure working level--any combination of daughter products (RaA, RaB, and RaC,) which produce 1.3.times.10.sup.5 million electron volts (MeV) of alpha energy is one liter of air. This indirect method reduces or eliminates significant errors caused by plate out of the particulates when they are drawn through small orifices necessary when making radioactivity measurements on the collection side of the filter. The final step of our method is to evaluate the inherent error from the beta particle emitting method and provide an optimized error map.
The reference mentioned to Lamb teaches the use of a counter for airborne beryllium with an alpha source filter that reacts with the beryllium particles to produce carbon--12 atoms, gamma rays and neutrons. Radiation detectors detect gamma photons and/or neutrons and convert them into electrical pulses. The rate of these pulses corresponds to the amount of beryllium present in the air. In the Riel patent, a solution containing the radioactive isotope is pumped through a filter which has a radioactive isotope extracting resin. A detector and spectrometer than detects and counts the radioactive isotopes on the filter.
In addition to the foregoing prior art references U.S. Pat. No. 3,056,886 (Glaude et al) disclose a method wherein radon is absorbed by an activated carbon and then counting the beta particles emitted by radium C in the carbon.
The United States Bureau of Mines Report of Investigations (RI) 8237 published August 1977 entitled "Continuous Working Level Measurements Using Alpha or Beta Detectors" authored by us contains more details and background information relating to this invention and is specifically incorporated herein by reference.
Although much of the structure to perform the preferred embodiment of our invention shares a degree of common elements with the mentioned and known prior art, none describes or discloses the method set forth herein. In particular, none of the known prior art discloses a method for continuously measuring and determining the working level exposure to airborne daughters of the gas radon-222 in which the alpha energy concentration is determined by measuring the beta count and then correcting for errors in the same.